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Bonding to Existing Concrete

posted by Bob Monday, November 22, 2010 at 12:00 PM

Fact: Fresh wet concrete does not normally bond well to existing dry concrete. Do you remember elementary school where one of the subjects on which you were graded was “plays well with others”? Concrete would have gotten an F. There is nothing in basic portland cement that will act as a bonding agent. Portland cement concrete works well in mass and provides great compressive strength but not bond.

Concrete is marvelous stuff but in time it will deteriorate. When it does you either have to patch it or replace it. Assuming that it is structurally sound the least expensive alternative is to patch it. However patching it requires some attention to detail or your patch will not last. So that you don’t waste too much time or money we should probably discuss what “structurally sound” means. If your sidewalk has either heaved or dropped at almost every joint, repairing it will not provide a long term solution. The slabs are likely still moving. If your slab has so much sand and gravel on the surface that despite sweeping and sweeping and squirting and squirting it just keeps coming back, don’t waste your time on repairs. If you have multiple cracks that run so deep that they appear to run through the slab, a repair would only be temporary. The solution to all of these problems involves a jack hammer and bags of one of the Sakrete concretes.

Since this discussion is on the best way to bond concrete we will assume that your slab is good.

There are a variety of Sakrete concrete repair products available to fix concrete that has begun to deteriorate. However without good surface preparation, none of them are going to perform satisfactorily. All loose sand, gravel, dirt, leaves etc. must be removed. This can typically be done with a garden hose and a good nozzle. Tough areas may require a pressure washer or mechanical abrasion. The two toughest areas to cover are those with oil and tree sap. Both of these will work their way down into the concrete. Simply washing the surface isn’t sufficient. If the stains do not run too deep you can chip away the concrete using a hammer and chisel. Don’t forget the goggles (not just glasses) as this process will throw concrete all over the place. Also keep your thumb out of the way. If the spots are too large or too deep for this to be practical you may need a sealer to cover the stains before patching.

There are two basic methods for bonding a portland cement based product to existing concrete; 1) chemically and 2) mechanically.

Let’s discuss the mechanical approach first since it is really used in both approaches. The most effective way to ensure a really good bond is with a scratch coat. This is simply a very wet coat made up by mixing the repair product with water. Mix up a small amount of the repair material to a soupy consistency. You don’t need to measure the water-just turn the stuff into slop. Then, using a gloved hand or a rag, smear the material onto the area to be patched. Just think finger painting from kindergarten. The technique is about the same. Apply pressure to ensure that as much as possible is shoved into the nocks and crannies. You only need a thin coat. It is not necessary for this scratch coat to dry. By the time you get the repair material mixed it will be ready. Then mix up additional repair material to the proper consistency and apply over this thin scratch coat.

The chemical approach involved mixing up a liquid bonding agent that helps bond new concrete products to old. Products like Sakrete Top n Bond and Sakrete Flo-Coat already contain polymers that greatly improve the bond of portland cement and should NEVER be used with a liquid bonding agent. I know in America bigger is better but it just ain’t so with these products. Other products like Sakrete Sand Mix and Sakrete Fast Set Cement Patcher benefit from the use of a liquid chemical bonding agent such as Sakrete Bonder/Fortifier. When you use a liquid bonding agent, paint the bonder onto the existing concrete and allow it to dry until it is tacky. This usually takes only a few minutes. Then apply the repair material. Just as in the process described above, after the bonder has become tacky apply a scratch coat and then apply the repair material. The most effective way to ensure that the bonding agent gets into the existing concrete is to apply it directly using a brush or rag. It can be sprayed if you happen to have a sprayer. Although the directions say that you can use it as part of the mix water, direct application works better.

If you are doing a large area and a scratch coat isn’t practical you will need to spray the surface with water before you apply the repair material. On a warm day the existing concrete surface will be hot enough to suck the water our of the repair material. In addition some concretes are quite porous and will also rob water from your repair material. If too much water is lost into the old concrete there will not be enough water to hydrate all of the cement particles and a lower strength material will be the result.

There are some substances that concrete simply will not bond to. Paint, oil, glue from old flooring tiles are just a few. You must mechanically remove these materials if you want the job to last.

Once the job is complete you can do a quick check to see if the bond was successful. Wait at least 24 hours and then tap “gently” on the patch using a hammer or some other dull object and listen for a hollow echoing sound. If you just get a dull thud then the material has bonded well. If you get a hollow sound, the material has not bonded and will crack in time. Which means it is back to the beginning of today’s topic. Here is hoping your concrete work comes across as a dull thud (not like some of my party guests) rather than a hollow endeavor.



LG, it sounds like you are setting a post on top of a footing that has already been poured. The rebar help join the existing concrete to the new and give added strength. We suggest using Sakrete Fast Setting Concrete Mix for this type of application. After your rebar is in place and the post is level fill the hole 1/3 full of water and pour in the Fast Setting Concrete directly from the bag filling the hole to the top. If the surface is dry add more water. Excess surface water is not a problem. Slope top of mix to allow water to drain away from the post.
- Chris Technical Services
Friday, August 29, 2014 at 9:16 AM

Dave, Unfortunately this cannot be accomplished by using the Sakrete Flo-Coat. We would not have any recommended materials through Sakrete for this type of application.
- Chris Technical Services
Tuesday, August 26, 2014 at 4:30 PM

Dick, first you will need to address the cracks. Depending on where the cracks have originated and if expansion joints were used, you may find that these cracks will reappear. If expansion joints are not present then what is happening is the slab is creating its own expansion joints and will require creating expansion joints where the hairline cracks are present. Once you have cut the new expansion joints and cleaned the area free of any dirt, debris, loose material, paint, stain, etc. use Sakrete Flo-Coat Concrete Resurfacer to resurface the area. Be sure to honor the new and existing expansion joints.
- Chris Technical Services
Tuesday, August 26, 2014 at 2:59 PM

Kris K, the area needs to be clean and free of any dirt, debris, loose material, paint, stain, and sealers. The area should be properly prepared prior to the installation to ensure that the new concrete will bond to the existing surface. If the area only requires up to a 2” build-up you can use Sakrete Sand Mix. That topping material can be used down to ½”. The Sakrete Sand Mix can be modified by using Sakrete Bonder and Fortifier as an admixture (50/50 ratio with water) to achieve a superior bond with the existing slab. Finish off the last ½” using Sakrete Flo-Coat. Flo-Coat allows you to complete the surface from that last ½” to featheredge if needed. Keep the materials in the green stage together by applying them no more than 24 hrs apart. This allows the materials to cure out together. Allow 28 days for full cure before tiling the surface.
- Chris Technical Services
Tuesday, August 26, 2014 at 11:33 AM

HI, I need to raise the footing under a wood post 12" thus creating a horizontal cold joint. My question is what would be the proper way to prep the area to create the best bond. I'm also planning on drilling into the existing footing to add some rebar. The existing footing is 16"x16" and the new added will be 16"x16"x12"H
- LG
Monday, August 25, 2014 at 3:54 PM

I have a large slab (1000sq'). The slab is older with many hairline cracks and was never pitched properly so water ponds on it. It is however stable. It also has living space beneath it and some of the cracks leak through the slab. It is an exposed exterior slab in the Northeast on the shore and gets many freeze thaw cycles. It sounds like your Flo-coat would be the best product to try and remedy the ponding, seal the cracks and improve the appearance. If so, what would the life expectancy be of the resurfacing w/this product or would you recommend something else? I have done a lot of concrete work over many years and I have always been reluctant to 'stick' concrete to itself - especially in such an exposed and high use (foot traffic) area. The slab is at a multi million $ home, so I would need some reassurance this product will be successful and have a reasonable life expectancy. Thank you for your help.
- Dave S
Monday, August 25, 2014 at 9:18 AM

our patio 12x18 has developed hairline cracks, with the widest being maybe 1 / 8 of an inch. which product of yours should we use to repair and make the patio look perfect again? How much would we have to purchase? Would it require 2 coats? We want to do the complete surface to make it look uniform. Any other information will be greatly appreciated. Thanks Dick F
- small cracks in patio
Sunday, August 24, 2014 at 12:01 PM

I have a old basement concrete floor house built in 1925. I have a bathroom in basement that needs the floor leveled before I tile it. I know i need a good bond. What do you recommend and what material should I use to level it? Thanks
- Kris K
Tuesday, August 19, 2014 at 3:21 PM

Ed, Sakrete Flo-Coat is designed for horizontal surfaces when mixed to its flowable consistency. You can however apply a thin layer in a trowelable consistency to the vertical stair face by using 8 part powder to 1 part water ratio mixture.
- Chris Technical Services
Friday, August 15, 2014 at 2:58 PM

Can Sakrete Flo-Coat work on a verticle surface like concrete stair risers?
- Ed
Friday, August 15, 2014 at 12:38 PM



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